When utilizing high speed current limiting circuit interruption to interrupt the circuit current in the early stages of the current waveform, it is important that the movable contact arm size to be as small as possible to promote electrodynamic repulsion. A further approach to improving the dynamics of contact arm repulsion is to eliminate the heavy flexible braid conductor that connects the contact arm to the load terminal.
U.S. Pat. No. 4,931,603 entitled "Molded Case Circuit Breaker Movable Contact Arm Arrangement" describes a "braidless" connection between the movable contact arm and its support which does not require any flexible braid conductor and which is of a small size to promote electrodynamic repulsion. This Patent should be reviewed for its detailed description of the movable contact arm support.
When current limiting circuit interrupters are used within higher ampere-rated circuits, a small auxiliary flexible braid conductor is used to provide a parallel current path to deter pitting and such other corrosive electrical effects from occurring at the contact arm-contact support interface.
A further example of a braidless movable contact arm is found in U.S. Pat. No. 4,733,033. This Patent discloses the use of a spring having a planar configuration capable of holding the contact arm against its support posts with sufficient force to maintain electrical contact during overcurrent conditions. However, when this design is used within higher ampere-rated current limiting industrial circuit breakers, a parallel current path should be connected between the movable contact arm and the contact arm support posts to prevent the occurrence of arcing between the contact arm and the support posts under intense short-circuit overcurrent conditions.
The use of a smaller, lighter, flexible braid conductor to reduce the dynamic drag at the end of the movable contact arm, as described within U.S. Pat. No. 4,999,464, could cause wear on the end of the smaller, flexible braid under accelerated test conditions. This is believably due to the friction that occurs between the internal circuit breaker components and the end of the flexible braid that is directly welded or brazed to the end of the movable contact arm.
U.S. Pat. No. 4,583,065 entitled "Electric Connection of Braids on a Circuit Breaker Terminal" describes an arrangement for attaching the opposite end of the braid conductor to the circuit breaker terminal supports.
U.S. patent application Ser. No. 07/764,287 filed Sep. 23, 1991 entitled "Molded Case Circuit Breaker Movable Contact Arm Arrangement" describes the use of an off-set welding tab at the point of contact between the braid conductor and the movable contact arm to eliminate the fraying of the braid conductor that could occur under extreme test conditions.
U.S. Pat. No. 5,217,111 describes a screw retainer used with the movable contact arm assembly to retain the load terminal screw within the assembly. When movable contact arms of increased thickness are used within higher ampere-rated circuit breakers, a corresponding larger contact arm support assembly is required to accommodate the increased thickness.
It would be economically advantageous to provide a single contact arm support assembly to accommodate movable contact arms of differing thickness and to eliminate the requirement for the separate terminal screw retainer.
Further economic advantage could be attained by eliminating the off-set tab and the extra welding operations that the tab requires.
One purpose of the invention is to describe a movable contact arm support assembly capable of accepting movable contact arms of differing thickness as well as incorporating a screw retainer within the contact support structure. An additional purpose of the invention is to eliminate the off-set welding tab.